1. Technical Field
The present invention relates to a surface-curved optical element used in transmitter and receiver for optical communication.
2. Prior Art
An optical transmitter incorporating the optical element of well known type will be discussed in reference with FIG. 8 of the accompanying drawings, in which reference numeral 1 designates a paraboloidal concave mirror and a light emitting source 2 is located at a focal point Fa of this paraboloidal mirror 1.
Reference numeral 3 designates the optical element in the form of a transparent cover having two boundary surfaces comprising spherical surfaces of which the spherical surface 3a defining the inner boundary surface and the spherical surface 3b defining the outer boundary surface are configured so as to be mutually in concentric relationship at a point A.
In other words, the transparent cover 3 is formed by a transparent synthetic resin plate of a uniform thickness. Said light emitting source 2 is located on an imaginary straight line Z corresponding to an optical axis of this transparent cover 3.
A light ray L24 emitted from the light emitting source 2 is reflected by the paraboloidal mirror 1 so as to be directed along the optical axis Z toward the transparent cover 3 in the form of a light ray L25 and enters the inner spherical surface 3a of the transparent cover 3.
The light ray L25 having entered the spherical surface 3a at a point P1 is now refracted so as to be oriented along extension of a straight line connecting a focal point Fb and said point P1 of the spherical surface 3a in the form of a light ray L26 and this refracted light ray L26 then enters the outer spherical surface 3b of the transparent cover 3.
The light ray L26 is refracted again at a point P2 on the spherical surface 3b and exits the transparent cover 3 now in the form of a light ray L27.
With the optical transmitter as has been described above, an optical attenuation is enhanced as a function of a distance over which the optical communication is desired, since the light ray L27 exiting the transparent cover 3 becomes divergent. Accordingly, such optical transmitter can not reliably function in the optical communication over a relatively long distance.
More specifically, an angle .theta.1 at the point P1 must be equal to an angle .theta.2 at the point P2 in order to assure that the light ray L27 should be in parallel to the optical axis Z and, to meet this requirement, a normal line N1 to the point P1 must be in parallel to a normal line N2 to the point P2.
However, these two normal lines N1, N2 are straight lines passing the common centre A and therefore they become non-parallel straight lines intersecting each other at an angle of .theta.3 as indicated in FIG. 8 rather than extending in parallel to each other.
In a consequence, .theta.1.noteq..theta.2 and the light ray L27 becomes divergent.
While it may be possible to direct the light ray L27 emerging from the transparent cover 3 in parallel to the optical axis Z when said transparent cover 3 is formed by plano glass, use of such plano glass will inevitably standardize the external appearance of the transparent cover 3 and restricts a freedom of designing.